The invention relates to magnetic disk drive units for use in data processing systems. More specifically, the invention pertains to magnetic disks including servo positioning information and servo positioning systems for data track seeking and following operations.
A data storage system of the type herein described typically includes a circular magnetic recording and playback medium that rotates on a drive unit, at least one transducer or read/write head for transferring data signals to and from the magnetic medium, and a controller that either selects the transducer that transfers data or moves a positioner that aligns the transducer over a selected data track of the magnetic medium. The controller actuates the positioner which moves the transducer in response to servo data contained on the recording medium. That portion of the recording medium not containing servo positioning data is dedicated to storage information, or processed data, that are generated by the data processing system.
The type of controller employed characterizes the generally known classes of disk storage system, namely, fixed head drive systems and movable head drive systems. In a fixed-head disk drive system, at least one transducer is positioned over the path of each concentric data track, whereas in movable head disk drive systems, a single set of heads is moved by a servo controller from track to track in response to commands from the data processing system. The expense of head duplication in fixed-head systems, in certain applications, is justified because there is no positioning delay, i.e. the delay encountered while the heads are being moved from one track location to another track location. On the other hand, however, where positioning delay is tolerable, the more economical movable head drive systems are desirable provided track seeking and following tasks can be performed accurately and timely while attaining a substantially high track density on the magnetic storage medium.
In movable head system disk systems the data disk used therein contains prerecorded servo data, or positioning information, on the surface of the magnetic recording medium. That servo data may be interleaved with work data on each of the concentric tracks, or all servo data may be contained on a dedicated servo surface of the recording medium while the work data is contained on the remaining surfaces of the recording medium. In a magnetic disk of the former type, servo positioning information can be advantageously read by the same transducer that reads the storage data. The arrangement of the of servo positioning information may be varied according to the particular track seeking and track following algorithm employed by the disk system. One such arrangement is disclosed in U.S. Pat. No. 4,101,990 wherein both servo and storage data are contained on each surface of the disk. A transducer is positioned over a data track centerline by a controller using a null detector that detects null signals generated by boundaries of adjacent plused servo signals of opposite polarity.
In another type of drive system, such as that disclosed by U.S. Pat. No. 4,032,984, coarse positioning data is recorded in one servo sector and fine position data is recorded in another servo sector. The servo controller uses the fine positioning data to follow a designated data track and uses the coarse positioning data to seek a designated data track within a band of several data tracks. During track seeking operations, the servo controller interprets information contained in three distinct and successive cells of a servo sector which provides eight unique location indications within an eight track group. When the transducer is moved to a new track, the information at an initial position is compared with information at a final position. The new track position is then determined by the comparison. The controller then decrements a difference counter in response to this information.
Another servo positioning system, described in IBM Technical Disclosure Bulletin Volume 13, No. 10, March 1971, p. 3161 illustrates servo signals used for positioning a read/write head which can be positioned over a selected data track by employing a secondary head coupled to the read/write head. The secondary, or servo, head generates a position error signal derived from a composite signal dependent upon the position of the servo head with respect to adjacent servo signals which are the inverse of the other shifted in phase by 90.degree..